Corrosion resistant ferrous alloy comprising chromium, nickel, manganese and copper



Patented June 12, 1934 CORROSION RESISTANT FEBIROUS ALLOY COMPRISING CHROMIUM, NICKEL, MAN- GANESE AND COPPER Percy A. E. Armstrong, Beverly Hills, Calif.

No Prawns.

8 Claim.

This application relates to ferrous alloys containing chromium and nickel, which alloys also contain other ingredients in novel proportions which give to the alloys unusual and valuable characteristics not heretofore attainable. This application is a continuation in part of my earlier application, Ser. No. 532,125, filed April 22, 1931.

may run upward to as much as though for purpose of economy it ordinarily will be kept below about 18%. At the low end at least about 8% of nickel is the minimum desirable quantity, go but in giving this limit I mean to imply that melting ranges may be permitted. such as are found in the commercial production of the 18 and 8 alloy where the nickel usually ranges upward from a point above 7%. Ordinarily when the carbon is low the nickel will be increased with the chromium but this is not always the case. Alloys of this general class have very valuable characteristics as regards corrosion resistance and toughness, but it is recognized that they have certain weaknesses as, for example, in their resistance to corrosion by sulphuric acid, and particularly to what is known as intergranular corrosion evidenced by the copper sulphate test, especially when the alloy has been reheated to comparatively low temperatures in the order of 1250 F. or an equivalent phase change brought about by cold working. Also it is recognized that while these alloys are austenitic or of the gamma phase when heat-treated, that is, quenched from a high temperature, and therefore'non-magnetic, they are not stable in this condition and when they are transformed into the magnetic condition (as by cold working) their corrosion resistance will be materially reduced. Also due to their tendency to develop mechanical martensite (involving a change of phase to alpha) they ofler great resistance to machining.

I have discovered that if there is added to a ferrous alloy of the chromium-nickel type, properly proportioned and adjusted amounts of copper and manganese, the alloy will retain the ready forgeability in excess of the corresponding iron-chromium-nickel alloy and will be stable austenite (that is so-called stable gamma) so that it will remain non-magnetic even to the Application December 14, 1933, Serial No. 702,417

point where it is cold worked sufficiently to give a wear-hard surface. This same stability of the austenitic or gamma phase will prevent the development of mechanical martensite or alpha phase so that the alloy can be readily turned, sawed or otherwise machined.

It will also be found that the resistance of my alloy to the action of sulphuric acid is increased phenomenally and its resistance to intergranular corrosion has, been enormously increased. This 65 latter feature is a very surprising factor for manganese certainly does not improve the intergranular corrosion and apparently lowers it, and the action of copper alone in this regard is very slight and practically negligible; copper and manganese additions will not produce my results unless the copper and manganese content have a definite relationship to one another. I

The improvement in intergranular corrosion first is observed at the point where the manganese is about equal-tothe copper and as the manganese proportion is increased becomes more and more noticeable. Thus decidedly improved results are obtained when the manganese to copper proportion is at least 4:3 and even a better no resistance is obtained when this proportion is 3g2 or higher. The copper must not be less than about 1% and preferably is over 1.3% and under about 3%. The manganese ordinarily will range from about 2% to about 8%. 'If resistance to intergranular corrosion is not of vital importance, the manganese range relative to copper may be wider, say, ranging from about equal to the copper up to about 12%, but the high'manganese will add to the expense and instead of giving any commensurate benefit will tend to give a predominant manganese result which is detrimental, though even up to approximately the figure of 12% manganese my alloy will show a resistance to sulphuric acid far greater than would be expected from the known effects of copper or manganese alone.

While the lower ranges of manganese within my proportions (that is, where the manganese approaches equality with the copper) have somewithout the manganese and copper and have the further advantage of definitely improved resistance to scaling at high temperatures, say between 1600 F. and 1800" F., and this is true even though the silicon content is kept low. Thus even without high silicon, steel embraced within the pres-, 0

ent invention has definitcadvantages and uses in the field oi valves for internal combustion enbest analysis will include copper from 1%% to the manganese may be from 2% to 4%and about-3% and manganese from 4% to 6%. H inter-granular corrosion is not so important and resistance is to be had againstsulphuric or nitric acids or against scaling at high temperatures,

copper from l /2% to about 2.5%.

As the manganese increases above 8%, it will be found that maximum precipitation by drawing at about 1250 F. for long periods will prothe .duce slight magnetism, but these same alloys v creasing the silicon or aluminum content somewill beaustenitic when quenched from 1950 F. to 2000 F. and will remain stable austenite even under severe cold work. Above about 8% 01 manganesethere is a. slight falling oil of resistance to intergranular corrosion, even though the copper is kept to the upper limits; as is desirable with the higher manganese, and for this purpose the manganese should not be more than 3 times the copper. It is not feasible to increase the copper proportionate to increases in the man- I ganese because ii the copper is brought above sistance to intergranular corrosion. In such case the manganese should not exceed substantially four times the copper.

The following are specific this class:

- Carbon Chromium Nickel Manganese Copper Silicon .10 I 17.46 8.25 4.01 2.9 .31 .08 17.46 s25 sea a9 .39 .10 17 52 8.25 am 29 .30 .10 17135 5117 3.09 1.3 .20 .10 17.14 s17 29s as .39 .12 18.83 7.42 3.84 2.2 42 .11 1a1s 7.39 2.24 1.9 .38

In addition to these analyses the'tollowing give approximate analyses showing other relationships of chromiumand nickel. It is understood that in each of these the carbon content is preferably low:

Chromium Nickel Manganese Copper l2 l0 6 3- 15 8' 4 2 15 8 4 3 l5 l0 4 3 l5 8 8 3 20 9 6- 3 25 15 8 3 28 17 Y 7 2.5 30 25 8. 3

High melting point elements such as tantalum, columblum, vanadium. molybdenum, tungsten, titanium and zirconium may be used in small quantities to give varying physical characteristics normally resulting from their use. Also the inclusion of fractional percentages of sulphur or selenium may somewhat improve the machineanalyscs oi alloys of ability. "Ihecarbon content of my alloy ordinarily is low-preferably under .15% but may range up to about 30% and will only exceed this figure it intergranular corrosion and iorgeability are to be sacrificed in the interest of hardness as in the case 01 valves, in which case a substantially higher carbon content may be employed. For the purpose of this alloy the silicon will ordinarily be low, as within the ordinary ranges encountered in steel making practice and under about 1.00%. 1

My alloy has a tightly adhering scale quite diiierent from the light, flufiy type common to the regular chromium-nickelalloys oi the usual type referred to, and this scale gives to my alloy excellent resistance to elevated temperatures up until a point of about 1600 to 1809 is reached when the emciency begins to fall oil, due to the progressive scaling action, though even at these temperatures the resistance to intcrgranular corrosion is still of great benefit. However, by inhavinga scale which will remain practically unchanged even at very high temperatures ranging up to the'order of about 1800 F. to zoom F., provided the temperature does not approach within about 200 F. of the transformation point of the particular alloy. Further, if the silicon or aluminum content is not unduly increased there will be an improvement in resistance to intergranular corrosion as compared with a corresponding steel not having the manganese and copper content so that a. valve of my steel with the increased silicon or aluminum content is particularly advantageous for use in high compression engines where it must withstand the action of gases which result from the combustion of gasoline which has been treated with tetraethyl lead. However, it must be borne in mind that even at these high temperatures the resistance of the valve is dependent upon the maintenance of the non-porous scale,- and if this is ruptured a high silicon or aluminum content will make the valve subjected to intergranular corrosive action, and it operated at lower temperatures a high silicon content may readily set up a mechanical weakness due to heavy grain boundary precipitate; whereas, when the silicon is low the alloy at these lower temperatures of about 1000 F. to 1500 F. does not sufier from a grain weakening precipitate, although a precipitate does exist or a very unusual type, that is highly-non corrosive. For uses where inter-granular. corrosion resistance or weakness is not of importance, but high temperatures'are to be met, the silicon may range up to about 3%; 7

In the foregoing analyses the iron content is not included, but it is of course understood that subject to the additions of comparatively small quantities of various elements as stated above and subject to the presenceof small quantities of other elements such as theimpurities found in steels, the balance is iron. Accordingly to cover this situation, I may state that in addition to the elements named in the analyses the balance is substantially all iron.

What I claim is:

'1. A stable surface ferrous alloy characterized by its resistance to. sulphuric acid, resistance to phase chan'gewhen quenched from high temperature and submitted to severe cold work, and

resistance to intergranularcorrosion when subiected to maximum precipitation, and .of good forgeability and machineability, comprising chromium about 12% to nickel ranging upward from at least about 8% to copper between about 1% and 3%, manganese between about 2% an'd-12% but at least equal to the copper, but not substantially more than iour times the copper, silicon less than 3%, and the balance substantially all iron.

2. An alloy as specifled'in claim 1 in which the nickel content ranges from at least about 8% 3. An alloy as specified in claim 1, in which the copper content is between 1.3% and about 3% and'the manganese content is between about 2% and 8% andis substantially in excess of the copper.

4. Aierrous alloy characterized by its stability in the gamma phase, its resistance to intergranular and acid corrosion and its Iorgeability and machineability, comprising chromium about 12% to 25%, nickel ranging up i'rom'at least about 8% to 18%, copper between about 1% and 3%, manganese about 2% to 8%, the ratio of manganese to copper being not less than 4 3 or not over about 30%, and silicon less than 1%,

the balance being substantially all iron.

6. Aierrous alloy characterized by its resistanceto acid corrosion or intergranular corrosion, by its machineability and iorgeability and its stable non-magnetic condition comprising chromium about 18%, nickel about 8%, copper about 1.5% to 3%, manganese about 4% to 6%, carbon not substantially above about .15% and silicon less than 1%, the balance being substan- 8 tially all iron.

7. A ferrous alloy characterized by its resistance to the action of sulphuric or. nitric acid, by its machineability and forgeabiiity and its stable non-magnetic condition, comprising chromium about 18%, nickel about 8%, copper about 1.5% to 2.5%, manganese about 2% to 4% and in excess of the copper, carbon not substantially above about 15% and silicon less than 1%, the

18%, copper between about 1% and about 3%,

manganese between about2% and 12% in excess of the copper but not substantially more than four times the copper and the balance being substantially all iron.

PERCY A. E. ARMSTRONG. .110 

